1. Field of the Invention
The present invention relates to nickel-iron-chromium alloys containing molybdenum for the purpose of providing resistance to pitting and crevice corrosion.
2. Discussion of the Related Art
Certain ferrous alloys including INCOLOY® alloy 25-6MO (hereinafter referred to as “alloy 25-6MO”) are particularly useful for their exceptional resistance to many corrosive environments. INCOLOY® is a trademark of the Special Metals group of companies. Alloy 25-6MO nominally contains by weight percent 25 nickel, 20 chromium, and 6 molybdenum. Examples of such corrosion resistant alloys are disclosed in U.S. Pat. No. 4,545,826 as containing by weight percent 20-40 nickel, 14-21 chromium, 6-12 molybdenum, maximum of 2 manganese, and 0.15-0.30 nitrogen. These alloys are annealed at relatively high temperatures, namely, over 2100° F. (1149° C.), typically about 2200° F. (1204° C.).
These nickel-chromium-molybdenum alloys are particularly suited for use in chemical and food processing, pulp and paper bleaching plants, marine and offshore platforms, salt plant evaporators, air pollution control systems, and various equipment for the power industry. These are aggressive aqueous environments which contain halides. Accordingly, the alloys formed into components of such systems must have good resistance to pitting and crevice corrosion. In addition, the alloys must have good processability since they are fabricated into a variety of intricate forms. Processability includes well-known hot forming techniques such as forging and rolling or other forming operations such as drawing and bending to mention a few. However, it is difficult to produce a nickel-chromium-molybdenum alloy with good processability because high concentrations of Mo, Cr and N which provide pitting resistance are also known to be detrimental to the processability of the alloy.
Accordingly, a need remains for a nickel-chromium-molybdenum alloy having improved corrosion resistance as well as improved processability.